The study of protein/protein interaction, as exemplified, e.g., by the identification of ligands for receptors, is an area of great interest. Even when a ligand or ligands for a given receptor are known, there is interest in identifying more effective or more selective ligands. GPCRs will be discussed herein as a non-exclusive example of a class of proteins which can be studied in this way.
The G-protein coupled receptors, or “GPCRs” hereafter, are the largest class of cell surface receptors known for humans. Among the ligands recognized by GPCRs are hormones, neurotransmitters, peptides, glycoproteins, lipids, nucleotides, and ions. They also act as receptors for light, odors, pheromones, and taste. Given these various roles, it is perhaps not surprising that they are the subject of intense research, seeking to identify drugs useful in various conditions. The success rate has been phenomenal. Indeed, Howard, et al., Trends Pharmacol. Sci., 22:132–140 (2001) estimate that over 50% of marketed drugs act on such receptors. “GPCRs” as used herein, refers to any member of the GPCR superfamily of receptors characterized by a seven-transmembrane domain (7TM) structure. Examples of these receptors include, but are not limited to, the class A or “rhodopsin-like” receptors; the class B or “secretin-like” receptors; the class C or “metabotropic glutamate-like” receptors; the Frizzled and Smoothened-related receptors; the adhesion receptor family or EGF-7TM/LNB-7TM receptors; adiponectin receptors and related receptors; and chemosensory receptors including odorant, taste, vomeronasal and pheromone receptors. As examples, the GPCR superfamily in humans includes but is not limited to those receptor molecules described by Vassilatis, et al., Proc. Natl. Acad. Sci. USA, 100:4903–4908 (2003); Takeda, et al., FEBS Letters, 520:97–101 (2002); Fredricksson, et al., Mol. Pharmacol., 63:1256–1272 (2003); Glusman, et al., Genome Res., 11:685–702 (2001); and Zozulya, et al., Genome Biol., 2:0018.1–0018.12 (2001), all of which are incorporated by reference.
The mechanisms of action by which GPCRs function has been explicated to some degree. In brief, when a GPCR binds a ligand, a conformational change results, stimulating a cascade of reactions leading to a change in cell physiology. It is thought that GPCRs transduce signals by modulating the activity of intracellular, heterotrimeric guanine nucleotide binding proteins, or “G proteins”. The complex of ligand and receptor stimulates guanine nucleotide exchange and dissociation of the G protein heterotrimer into α and βγ subunits.
Both the GTP-bound a subunit and the βγ dimer can act to regulate various cellular effector proteins, including adenylyl cyclase and phospholipase C (PLC). In conventional cell based assays for GPCRs, receptor activity is monitored by measuring the output of a G-protein regulated effector pathway, such as the accumulation of cAMP that is produced by adenylyl cyclase, or the release of intracellular calcium, which is stimulated by PLC activity.
Conventional G-protein based, signal transduction assays have been difficult to develop for some targets, as a result of two major issues.
First, different GPCRs are coupled to different G protein regulated signal transduction pathways, and G-protein based assays are dependent on knowing the G-protein specificity of the target receptor, or require engineering of the cellular system, to force coupling of the target receptor to a particular effect or pathway. Second, all cells express a large number of endogenous GPCRs, as well as other signaling factors. As a result, the effector pathways that are measured may be modulated by other endogenous molecules in addition to the target GPCR, potentially leading to false results.
Regulation of G-protein activity is not the only result of ligand/GPCR binding. Luttrell, et al., J. Cell Sci., 115:455–465 (2002), and Ferguson, Pharmacol. Rev., 53:1–24 (2001), both of which are incorporated by reference, review other activities which lead to termination of the GPCR signal. These termination processes prevent excessive cell stimulation, and enforce temporal linkage between extracellular signal and corresponding intracellular pathway.
In the case of binding of an agonist to GPCR, serine and threonine residues at the C terminus of the GPCR molecule are phosphorylated. This phosphorylation is caused by the GPCR kinase, or “GRK,” family. Agonist complexed, C-terminal phosphorylated GPCRs interact with arrestin family members, which “arrest” receptor signaling. This binding inhibits coupling of the receptor to G proteins, thereby targeting the receptor for internalization, followed by degradation and/or recycling. Hence, the binding of a ligand to a GPCR can be said to “modulate” the interaction between the GPCR and arrestin protein, since the binding of ligand to GPCR causes the arrestin to bind to the GPCR, thereby modulating its activity. Hereafter, when “modulates” or any form thereof is used, it refers simply to some change in the way the two proteins of the invention interact, when the test compound is present, as compared to how these two proteins interact, in its absence. For example, the presence of the test compound may strengthen or enhance the interaction of the two proteins, weaken it, inhibit it, or lessen it in some way, manner or form which can then be detected.
This background information has led to alternate methods for assaying activation and inhibition of GPCRs. These methods involve monitoring interaction with arrestins. A major advantage of this approach is that no knowledge of G-protein pathways is necessary.
Oakley, et al., Assay Drug Dev. Technol., 1:21–30 (2002) and U.S. Pat. Nos. 5,891,646 and 6,110,693, incorporated by reference, describe assays where the redistribution of fluorescently labelled arrestin molecules in the cytoplasm to activated receptors on the cell surface is measured. These methods rely on high resolution imaging of cells, in order to measure arrestin relocalization and receptor activation. It will be recognized by the skilled artisan that this is a complex, involved procedure.
Various other U.S. patents and patent applications dealing with these points have issued and been filed. For example, U.S. Pat. No. 6,528,271 to Bohn, et al., deals with assays for screening for pain controlling medications, where the inhibitor of β-arrestin binding is measured. Published U.S. patent applications, such as 2004/0002119, 2003/0157553, 2003/0143626, and 2002/0132327, all describe different forms of assays involving GPCRs. Published application 2002/0106379 describes a construct which is used in an example which follows; however, it does not teach or suggest the invention described herein.
It is an object of the invention to develop a simpler assay for monitoring and/or determining modulation of specific protein/protein interactions, where the proteins include but are not limited to, membrane bound proteins, such as receptors, GPCRs in particular. How this is accomplished will be seen in the examples which follow.